I have the Sony Touch - got it as a gift. I like it well enough, but apparently it's the only one with no wireless access. That's not a huge deal to me, just something to be aware of. I'd almost say that it's better off without, since you don't have the distraction you do on a fully functional computer, of always wanting to check email, TE, Facebook, the weather, etc., etc....
It's not backlit, which supposedly enhances readability in potentially glare-y environments, but it means you have to have external lighting whenever you'd have to have it to read paper. I will say that when there's adequate light, it's
very readable - much more so than a computer screen.
It's really nice being able to scrawl notes in the margins, highlight pull quotes, and get back to them whenever you want.
Formatting can be an issue with books you don't buy through the Sony store, but I'm reading that the manufacturers and publishers are working on an industry standard format. If and when that happens, it should just be a flash update for all of the readers. It really doesn't make them less readable, just chapter headings, etc., can look weird.
Public domain Google books work fine on the Sony, according to my dad who reads a LOT of them that way. I haven't tried that yet. But there are a lot of books in that archive, and they're adding more all the time.